Automotive mufflers are well known for their ability to reduce noise generated by automotive internal combustion engines; however, in addition to the type of noise reduced by mufflers, there is noise emanating from the tailpipe which is generated downstream of the muffler. The cause of such noise has not heretofore been fully understood; and efforts to reduce such noise in a cost effective manner without creating engine performance deterioration have not been successful.
Diffusers have sometimes been useful for reducing noise by reducing fluid exit velocity. Webster's New Collegiate Dictionary (1981) defines diffusers as "a device for reducing the velocity and increasing the static pressure of a fluid passing through a system". As hereinafter used in this specification and appended claims, "diffuser" shall mean a fluid carrying passage which has an inlet cross-sectional flow area less than its outlet cross-sectional flow area, and which decreases the velocity of the fluid in the principal flow direction and increases its static pressure.
If the walls of the diffuser are too steep relative to the principal flow direction, streamwise, two-dimensional boundary layer separation may occur. Streamwise, two-dimensional boundary layer separation, as used in this specification and appended claims, means the breaking loose of the bulk fluid from the surface of a body, resulting in flow near the wall moving in a direction opposite the bulk fluid flow direction. Such separation results in high losses, low pressure recovery, and lower velocity reduction. When this happens the diffuser is said to have stalled. Stall occurs in diffusers when the momentum in the boundary layer cannot overcome the increase in pressure as it travels downstream along the wall, at which point the flow velocity near the wall actually reverses direction. From that point on the boundary layer cannot stay attached to the wall and a separation region downstream thereof is created.
To prevent stall a diffuser may have to be made longer so as to decrease the required diffusion angle; however, a longer diffusion length may not be acceptable in certain applications due to space or weight limitations, for example, and will not solve the problem in all circumstances. It is, therefore, highly desirable to be able to diffuse more rapidly (i.e., in a shorter distance) without stall or, conversely, to be able to diffuse to a greater cross-sectional flow area for a given diffuser length than is presently possible with diffusers of the prior art.
The automotive industry has experimented with diffusers to reduce tailpipe noise, but has not been totally successful. Specifically, diffusers have been added to the outlet end of a conventional cylindrical tailpipe, with a perforated plate disposed downstream of the diffuser, transverse to the exhaust flow direction. Such diffusers have been of the conventional conical variety, transitioning the tailpipe from one diameter to another larger diameter in gradual fashion; and they have been of the "dump" diffuser variety, which provides a step change in the tailpipe cross-sectional area. While these diffuser/perforated plate combinations have sometimes proved effective in reducing noise, the back pressure created by the perforated plate and by diffuser stall has been unacceptable; therefore, these configurations have not been widely adopted for production.
It is well known in the gas turbine engine art to use a convoluted exhaust nozzle to reduce jet noise, as shown and described in U.S. Pat. Nos. 3,635,308; 4,117,671 and 3,696,617, for example. The device described in the '617 patent uses a convoluted ejector configuration to draw ambient air into the exhaust nozzle to mix with core engine exhaust gases. Such convoluted gas turbine engine exhaust nozzles have not been used on automotive exhaust systems despite the fact that such technology has been in the public domain for at least twenty years. This may be due to the fact that aircraft gas turbine engines and automotive exhaust systems are in non-analogous fields.